Potter Had Errors?

While everybody's going gaga over the new Harry Potter and the Half-Blood Prince, Mark had already pointed out some proofreading error in his blog (with sample image, thus spoiler alert especially for those who haven't destroyed the plastic covering of their newly bought copy of the novel). Is that really a grammatical error? I am no English guru, so I couldn't decide if it is (can somebody also verify that error?).

Anyway, blame me as a lowly muggle, but I am not much into Harry Potter books. Sorry to say that to the millions of Harry Potter fans out there. But I have read the first book of the series to see what the commotion is and I must say, I did like it. Though I couldn't help myself comparing it to reading a children's detective book series such as... Bobbsey Twins. You know - children involved, cheesy storyline, light and fluffy events that kindle to anyone still child at heart (Do not be misled, I am not saying I dislike these types of stories. They're not my current interests though).